The aim of the proposed research is to study the biochemistry and pathophysiology of the early stages plasma coagulation and kinin forming systems in man. Previous biochemical accomplishments of this laboratory have been focused on the pathways of activation of this system including the biochemistry of the interactions between prekallikrein activators (activated factor XII), prekallikrein and kallikrein, and high molecular weight kininogen. During the next grant period we will extend these observations to include exploration of the domains of prekallikrein and factor XI responsible for complex formation with high molecular weight kininogen. We will also elucidate the effect of high molecular weight kininogen in protecting kallikrein and factor XIa against inhibition by plasma protease inhibitors including Cl--inhibitor, Alpha1-antitrypsin, antithrombin III, and Alpha2-macroglobulin. Kinetic, molecular and immunochemical approaches will be used. Clinical investigation will attempt to improve existing functional assay for components of the contact system and to development of radioimmunoassay for kallikrein inhibitor complexes. These improved assays will be applied to study hypotensive endotoxemia, myocardial ischemia, nephrotic syndrome, rocky mountain spotted fever and transient ischemic attacks in man. We will also seek to determine what contact factors or protease inhibitors are present in platelets as well as subcellular localization and mechanism of release. We will explore whether plasma kallikrein or XIa can stimulate platelets and whether these components bind to platelets. This combined biochemical, immunological and clinical approach should further our understanding of this important system in host defenses as well as in the pathogenesis of disease states.